PROJECTSUMMARY Cells and organisms must coordinate their metabolic activity with changes in their nutrient environment. This coordination is achieved via the signaling networks that integrate local and systemic nutrient inputs and relay nutrient status to the control of cellular anabolic and catabolic processes. This task canbe carriedoutby the RAS-RAF-MEK-ERK cascade, a signaling system that is commonly activated by various growth factors and oncogenicevents.Inresponsetoamitogenfactorsuchastheepithelialgrowthfactor(EGF),ERKisactivated and promotes cell proliferation and differentiation by regulating activity of transcription factors involved in cell cycleprogressionandproliferation.However,muchlessisunderstoodabouthowERKsignalingdirectlycontrols metabolicprocesses.TargetingthekinasesRAF,MEKorERKiscurrentlyastrategyemployedtotreatseveral diseasesincludingcancer,type2diabetes,metabolicdisordersandneurodegeneration,howevermechanisms of resistance often occur. Therefore, elucidating the downstream targets of ERK and more specifically the molecular mechanisms by which ERK signaling drives metabolism is of great interest in order to identify new therapeutic strategies against ERK driven disease. Recently we discovered that the mechanistic target of rapamycin complex 1 (mTORC1) stimulates synthesis of purines and pyrimidines de novo through different molecularmechanisms.Nucleotidesplayacentralroleinmetabolismatafundamentalandcellularlevel.Purine andpyrimidinebasescanbesynthesizeddenovoorrecycledthroughthesalvagepathways.Nucleotidescarry packets of chemical energy (e.g. ATP, GTP) throughout the cell to the many cellular functions that demand energy,whichinclude:synthesizingnucleicacids,proteinsandcellmembranes.Underthisproposal,wepropose tostudytheinfluenceofERKsignalingonnucleotidesynthesis.WehaveidentifiedthatERKsignalingstimulates de novo purine synthesis in various settings through posttranslational modification of the enzyme PFAS (phosphoformylglycinamidinesynthase)whichbelongstothedenovopurinesynthesispathway.Weproposeto dissect the molecular mechanisms underlying this regulation (Specific Aim1). We will determine the role of the ERK-PFASaxisinthecontrolofcellgrowth(SpecificAim2).Furthermore,wewilldeterminetheimplicationof thisregulationinERK-mediatedbiologyanddisease(SpecificAim3).Thus,theoverallgoalofthisproposalisto decipher the molecular mechanisms by which ERK controls de novo nucleotide synthesis in normal and pathological settings. We anticipate that the proposed studies will yield new insights into how nucleotide synthesisisregulatedbyERKandwilluncovertherapeutictargetstoperturbERK-mediateddisease.